In metals, free electrons with an equilibrium electron density exhibit collective longitudinal density oscillations referred to as plasmons having an eigenfrequency called the plasmon frequency. The presence of a boundary formed by a surface gives rise to a further mode of oscillation, namely surface waves forming surface plasmons. Surface plasmons thus are collective oscillations of free electrons at the surface.
Surface plasmons at an interface of a metal with a dielectric can couple with a propagating electromagnetic field (photons) giving rise to surface-plasmon polaritons. These surface-plasmon polaritons propagate along the surface. Thus, interaction of light with electromagnetic waves in metal structures gives rise to electromagnetic fields arising from surface plasmon polaritons. The strength of the electromagnetic field associated with a surface-plasmon polariton decreases exponentially with increasing distance from the surface, both inside and outside the metal. Since surface plasmon polaritons propagate along the boundary of the metal and an external dielectric medium, they are sensitive to any change of the boundary, such as the adsorption of molecules on the conducting surface.
Metal plasmonic structures have been explored for application in waveguides, sensors, and modulators. Due to enhanced field strength, which is a typical property of surface plasmon polaritons, surface-plasmon-based biosensors for sensing of biomolecules promise an extremely high sensitivity, which is suitable for detection of even a single biomolecule arranged in proximity to the surface. Therefore, surface-plasmon-based biosensors seem ideally suited for lab-on-chip applications.
Due to the plasmon frequencies in metals, the operating frequency of known metal plasmonic structures is in the visible spectral range. The decay length governing the decrease of the field strength of the electromagnetic field of the surface plasmon polaritons in a dielectric fluid volume above the metal surface (herein called the sensing volume) is of the order of nanometers, thus, aiding in near field sensing of biomolecules up to the order of nanometers.
In order to be able to identify and distinguish between different biomolecules, spectroscopic measurements are often necessary, requiring an availability of surface-plasmon polaritons covering a frequency spectrum of interest. Tuning the plasmon frequency of the plasmons, however, is extremely difficult in known metallic structures. Hence, establishing a spectroscopy platform using plasmonic structures remains an open task.